1. Field of the Invention
The present invention relates to a ballast for a discharge lamp, and more particularly to an electronic ballast suitable for operating the discharge lamp utilized as a vehicular headlamp.
2. Description of the Prior Art
It is known that the discharge lamp will flicker repeatedly when coming near the end of the lamp life, resulting in an unreliable lamp operation as well as posing a stress to the ballast. In order to avoid this problem, U.S. Pat. No. 5,880,563 proposes to cease operating the lamp upon detecting the end of the lamp life. In this patent, the lamp flickering is monitored so as to determine the end of the lamp life when the number of the flickering out, i.e., unforced extinction of lamp exceeds a predetermined number. However, the scheme allows the lamp to flicker for some time period before ceasing the ballast, leaving an unstable lamp operation for some time period between the initial lamp extinction and the actual ceasing of the ballast. Thus, unreliable lamp operation still continues at the end of the lamp life, detracting from the safe lamp operation as well as still posing a stress to the ballast.